Generally in the manufacture of paper, a web of paper is formed on a drainage wire and passed through a plurality of press rolls whereon further water is extracted and then onto heated drier drums. In modern high speed machines use of open draws wherein the web is unsupported over short spans through the system are avoided, if possible, at least until such time as the web strength is sufficient to reduce the possibility of web breakage to a minimum. Preferably a web will be continuously supported through the machine to even better insure against the possibility of a breakage.
In the drier section the web is generally carried by a pair of felts, one felt wrapping the upper drums of the drier section and the second felt wrapping the lower drums of the drier section with the web passing back and forth between upper and lower drums and being transferred from the upper felt to the lower felt as it passes up and down over the various drums. In this manner the web is maintained in direct contact with both the upper and lower drier drums i.e. between the upper felt and and the peripheral surface of the upper drums and between the lower felt and the peripheral surface of the lower drums.
Obviously, this system results in a significant number of transfers since the web travels back and forth between the upper and lower drums and must transfer back and forth between the upper and the lower felts between each roll of the sequence with such transfers normally being in the form of open draws.
It is also known, particularly in the first drier section immediately following the press section of the paper machine, to utilize a single felt to carry the web through the first section until it increases in dryness and in strength so that the odds of a breakage are significantly reduced. However, when the web is carried on a single felt if it is sandwiched between the felt and the outer surface of the top drums it will be on the outside of the lower drums or vice versa. Thus on the drums where the web is outside, the drying efficiency is relatively low since the felt is introduced between the hot drier drum surface and the web to be dried and acts as an insulator. Furthermore there are problems in carrying the web around the drier drum when the web is on the outside of the felt and special means should be provided to insure that separation does not occur or alternatively the drier, and thus the paper machine, must be operated at a relatively low speed which defeats the purpose of the concept.
One arrangement wherein the web is carried on a single felt through the drier section is shown in Canadian Pat. No. 1,120,259 issued Mar. 23, 1982 Kankaapaa. In this arrangement the web travels with the felt 20 around a plurality of drier drums and is sandwiched between the felt and the drier drum on alternate drums and is on the outside of the felt on the intermediate drums. Suitable means such as a top felt may be used to hold the web against the carrying felt on the intermediate drums.
It is also known to use blow boxes employing Coanda nozzles to cause flow along the surface thereof and to induce a negative pressure allowing one side of the felt in the drier section. This technique is used to maintain contact between the web and the drier felt to permit higher speed operation by preventing or reducing the air entrainment with the felt and reducing the pumping effect of the drum so that the web tends to stay on the felt when traversing drums equivalent to the intermediate drums described hereinabove, i.e., where the felt is sandwiched between the web and the drier drum. It is also known to use suction boxes for a similar purpose as taught for example in Canadian Pat. No. 1,171,650, issued July 31, 1984 to Veddenpaa and in U.S. Pat. Nos. 4,359,827 and 4,359,828 issued Nov. 23, 1982 to Thomas.
Some papers mills are drier limited. In other words the drying capacity of the drier drums is what limits the maximum speed of the machine and thus if some means could be provided for increasing the drier capacity the machine could operate at higher speed and increased production. Generally such an increase in capacity can only be obtained by adding a drier drum between the presses and the first drier, for example in the form of a baby drier drum however due to the restriction in space little extra drying capacity can be provided. While such an addition could increase slightly the drying capacity of the equipment it does not permit a major improvement in speed. In many such driers there is sufficient space above the drier to accommodate further drums however the threading technique utilized in conventional felt wraps does not lend itself to stacking drier drums more than the conventional two-high arrangement.
The concept of carrying a web through a drier section on a single felt and providing turning rolls therebetween so that the web being dried is always in contact with the surface of the drier drum, i.e., between the felt and the drier drum has been known for many years, as shown for example, in Canadian Pat. No. 309,086, issued Mar. 3, 1931 to Vedder. In the particular concept illustrated in this patent the web being dried is separated from the felt between each of the drier drums in the sequence so that the felt takes one path and the web being dried another path spaced from the felt between each of the drier drums.
It has also been proposed to carry the web on the felt around the turning roll so that the felt and web travel together through the various nips with the felt always on the outside trapping the web between the drier drum and itself. In this particular concept the felt travels from the bottom of one stack to the top of the next adjacent stack so that the felt travel is down one stack, up to the top of the next and then down the following stack in order to permit broke removal.
As far as Applicant is aware neither of the above arrangements for carrying the web in contact with each of the drier drums in the sequence has been used to date. The earlier concept as described in Canadian Pat. No. 309,086 clearly has a feeding problem, a broke disposal problem and is limited to very low speeds whereas the more recent arrangement would probably permit broke disposal but limits severely the speed at which the drier may be operated since the web will not stay with the felt as it traverses the turning rolls at high speed.